[ELF] Deduce emulation from input files if -m is not given

elf/main.cc

@@ -103,6 +103,7 @@ void read_file(Context<E> &ctx, MappedFile<Context<E>> *mf) {
 template <typename E>
 static i64 get_machine_type(Context<E> &ctx, MappedFile<Context<E>> *mf) {
   switch (get_file_type(mf)) {
+  case FileType::ELF_OBJ:
   case FileType::ELF_DSO:
     return ((ElfEhdr<E> *)mf->data)->e_machine;
   case FileType::AR:
@@ -122,6 +123,17 @@ static i64 get_machine_type(Context<E> &ctx, MappedFile<Context<E>> *mf) {
   }
 }
 
+template <typename E>
+static i64
+deduce_machine_type(Context<E> &ctx, std::span<std::string_view> args) {
+  for (std::string_view arg : args)
+    if (!arg.starts_with('-'))
+      if (auto *mf = MappedFile<Context<E>>::open(ctx, std::string(arg)))
+        if (i64 type = get_machine_type(ctx, mf); type != -1)
+          return type;
+  Fatal(ctx) << "-m option is missing";
+}
+
 template <typename E>
 static MappedFile<Context<E>> *open_library(Context<E> &ctx, std::string path) {
   MappedFile<Context<E>> *mf = MappedFile<Context<E>>::open(ctx, path);
@@ -341,6 +353,10 @@ static int elf_main(int argc, char **argv) {
   std::vector<std::string_view> file_args;
   parse_nonpositional_args(ctx, file_args);
 
+  // If no -m option is given, deduce it from input files.
+  if (ctx.arg.emulation == -1)
+    ctx.arg.emulation = deduce_machine_type(ctx, file_args);
+
   // Redo if -m is not x86-64.
   if (ctx.arg.emulation != E::e_machine) {
     switch (ctx.arg.emulation) {

elf/mold.h

@@ -1264,7 +1264,7 @@ struct Context {
     bool z_relro = true;
     bool z_text = false;
     u16 default_version = VER_NDX_GLOBAL;
-    i64 emulation = EM_X86_64;
+    i64 emulation = -1;
     i64 filler = -1;
     i64 spare_dynamic_tags = 5;
     i64 thread_count = 0;

test/elf/emulation-deduction.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+export LANG=
+set -e
+testname=$(basename -s .sh "$0")
+echo -n "Testing $testname ... "
+cd "$(dirname "$0")"/../..
+mold="$(pwd)/mold"
+t="$(pwd)/out/test/elf/$testname"
+mkdir -p "$t"
+
+# Skip if target is not x86-64
+[ "$(uname -m)" = x86_64 ] || { echo skipped; exit; }
+
+cat <<EOF | cc -o "$t"/a.o -c -xc -
+void _start() {}
+EOF
+
+"$mold" -o "$t"/exe "$t"/a.o
+readelf --file-header "$t"/exe | grep -qi x86-64
+
+echo OK